Method and Apparatus for Producing Strip Having a Variable Thickness

ABSTRACT

Disclosed is a method for producing strip having a variable thickness in its length direction, in particular a repetitive thickness variation, using a stand of a rolling mill, wherein the strip has a thickness D 1  when entering the mill stand and a thickness D 2  when leaving the mill stand. The speed of the strip when leaving the mill stand is used as an input to control the speed of the rolls in the mill stand. Also disclosed is an apparatus for producing strip having a variable thickness.

The invention relates to a method for producing strip having a variable thickness in its length direction, in particular a repetitive thickness variation, using a stand of a rolling mill, wherein the strip has a thickness D1 when entering the mill stand and a thickness D2 when leaving the mill stand. The invention also relates to an apparatus for producing such strip material.

Usually when rolling a strip from a thickness D1 to a thickness D2 the rolling speed of the rolls of the stand of the rolling mill is the reference speed for the rolling, which stand speed is controlled by a speed controller. The speed of the other mill equipment, such as the coiler and decoiler, is controlled using the stand speed as input. To keep tension in the strip entering and leaving the stand of the rolling mill between reference values, tension controllers measuring the tension in the strip change the reference rotational speed of the coiler and decoiler relative to the stand speed. The speed of the strip entering the mill and the speed of the strip leaving the mill will change accordingly.

However, when a strip has to be produced in which significant thickness variations in its length direction are present at short distance, such as a strip from which tailor rolled blanks are made, the thickness of the strip has to be changed fast and the change in the thickness D2 of the strip leaving the mill stand can be up to 50%. Due to the change in thickness the speed of the strip leaving the mill stand changes according to mass flow preservation. As a second effect, due to the change in thickness of the strip the motor speed of the mill stand will change temporarily due to a change in the required rolling torque. This change in mill speed affects the speed of both the strip entering and the speed of the strip leaving the mill stand. For instance when the thickness of the strip leaving the mill stand is reduced this change is felt, since at that moment in time more energy is required for rolling the strip. Resulting from this is a drop in the tension of the strip entering the mill stand and of the strip leaving the mill stand, which can only be accommodated by changing the upstream and downstream equipment speed in an anticipatory manner, and/or by using a dancer or looper or such-like equipment for both the strip entering and the strip leaving the mill stand. Changing the speed of the coiler and decoiler fast is difficult because of the high inertia of drives and the coiled strip present on the (de)coiler, and therefore dancers or loopers are used to maintain tension. This however results in a costly rolling mill configuration.

It is an object of the invention to provide a method for producing a strip having a variable thickness, which is easier to control than with the known method.

It is another object of the invention to provide a method for producing a strip having a variable thickness, for which the rolling mill configuration is less complex than the present configuration.

It is also an object of the invention to provide a rolling mill configuration for producing strip having a variable thickness which is simpler of construction.

According to a first aspect of the invention, at least one of these objects is reached by executing a method for producing strip having a variable thickness in its length direction, in particular a repetitive thickness variation, using a stand of a rolling mill, wherein the strip has a thickness D1 when entering the mill stand and a thickness D2 when leaving the mill stand, wherein the speed of the strip when leaving the mill stand is used as an input to control the speed of the rolls in the mill stand.

This method uses a control mechanism using the speed of the strip leaving the mill stand to control the speed of the rolls in the mill stand, which makes it more easy to control the rolling mill. The change in speed of the strip leaving the mill stand when the thickness of the strip leaving the mill is changed, is now used to control the change in speed of the rolls in the mill stand. The speed changes upstream of the strip leaving the rolling mill are thus more naturally a result of the change in speed due to the change in thickness of the strip.

Preferably the strip between the mill stand and equipment downstream of the strip is kept at a certain tension, the tension of the strip being used as an input for controlling the speed of the mill stand and/or the speed of the downstream equipment. Both the speed and the tension in the strip leaving the mill stand can now be used to control the speed of the rolls in the mill stand, the speed of the strip controlling the general speed of the rolls in the mill stand, and the tension in the strip being used to fine-tune the speed of the rolls in the mill stand. The tension in the strip can also be used to control the speed of downstream equipment such as a coiler.

According to a preferred embodiment the speed of the strip leaving the mill stand is kept approximately constant. In this way the control of the production of a strip having a variable thickness is simpler, since now the speed of the downstream equipment such as the coiler can be held approximately constant, with only small variations due to the tension in the strip leaving the roll stand. The constant speed of the strip leaving the roll stand also provides a natural change in the speed of the rolls in the roll stand when the thickness D2 of the strip is changed; for instance, when the thickness D2 of the strip leaving the roll stand is reduced and the speed of this strip is kept constant, the speed of the strip entering the mill stand must be reduced. This means that the rolling speed of the rolls in the mill stand must be reduced, which is in line with the higher rolling torque required for rolling the strip to the reduced thickness D2.

According to a preferred embodiment, the strip leaving the mill stand is given a repetitive thickness variation having a repeating profile length of at most 4 meter. From such a strip it is possible to cut profiled lengths from which tailor rolled blanks (TRBs) can be produced.

Preferably a program is provided for determining the thickness D2 in the strip, the program controlling the gap of the mill stand. This program thus is the input for producing the strip having a variable thickness, and the resulting strip leaving the mill stand provides the speed of the strip with which the speed of the rolls in the mill stand is controlled.

Preferably another program is provided for controlling the speed of the strip when entering the mill stand. This program can change the speed of the strip entering the mill stand when the gap of the mill stand is changed. Usually the speed of the upstream equipment is changed to change the speed of the strip entering the mill stand.

According to a preferred embodiment, a decoiler is used to supply the strip to the rolling mill and a coiler is used to take up the rolled strip, wherein a first tension measuring device is present to measure the tension in the strip entering the rolling mill and a second tension measuring device is present to measure the tension in the strip leaving the rolling mill, wherein a speed measuring device is present to control the speed of the strip leaving the rolling mill, wherein the speed and tension measured with the speed measuring device and the second tension measuring device are used as input to control the rolling speed of the rolling mill. In this way, the control of the rolling mill for rolling a strip having a variable thickness is easy and straightforward, since the control uses the speed and tension of the strip leaving the rolling mill, and the measuring devices for measuring this speed and tension are state of the art.

Preferably, the speed and tension measured with the speed measuring device, the first tension measuring device and the second tension measuring device are used as input to control the tension in the strip entering the rolling mill using a dancer or equivalent equipment between the decoiler and the rolling mill. In this way the same measurements together with the measurement of the tension using the first tension measuring device can be used to control the entrance speed of the strip in the rolling mill.

More preferably, the speed and tension measured with the speed measuring device, the first tension measuring device and the second tension measuring device are used as input to control the decoiling speed of the decoiler. All the important equipment upstream of the produced strip is thus controlled using these measurements.

According to a preferred embodiment the rolling mill used is a single stand rolling mill. Using a single stand rolling mill makes the control of the rolling mill easier, since a mill having two or more stands requires a very accurate control of the stands in relation to each other, or requires the use of loopers between the stands.

According to a second aspect of the invention an apparatus for producing strip having a variable thickness in its length direction is provided, the apparatus comprising a stand of a rolling mill, upstream equipment, downstream equipment, a tension measuring device for measuring the tension in the strip leaving the mill stand and/or a speed measuring device for controlling the speed of the strip leaving the mill stand, wherein a dancer or looper or equivalent equipment has been placed between the upstream equipment and the mill stand to accommodate changes in strip length, and wherein no dancer or looper or equivalent equipment has been placed between the mill stand and the downstream equipment.

With this apparatus it is possible to produce strip having a variable thickness in a well controlled way according to the first aspect of the invention. The apparatus is more simple than the known apparatus for producing strip having a variable thickness, because only one dancer or looper needs to be used.

Preferably, a further tension measuring device has been provided for measuring the tension in the strip entering the mill stand. The further tension measuring device can be used to control the dancer or looper or equivalent equipment between the upstream equipment and the mill stand and the upstream equipment, such as a decoiler.

The invention will be elucidated hereinafter.

As an example we use a single stand mill having an available maximum gap change speed of 20 mm/s. When the thickness D2 of the strip has to be changed over 0.5 mm, this change will thus last at least 0.025 seconds. Should this change in thickness be performed over a strip length of 25 mm, the maximum rolling speed of the rolling mill thus has to be 1 m/s. For this example, we neglect slip.

Conventional control aims at keeping the speed of the rolls in the rolling mill constant. However, due to slip (ration between exit strip speed and roll speed), which we assume to change between 1% and 6%, the exit strip speed (speed of the strip leaving the rolling mill) changes from 1.01 to 1.06 m/s. This results in a temporary reduction of the tension in the strip between the rolling stand and the coiler, which affects the deformation behavior in the roll gap and thus affects the strip thickness. With conventional tension control the coiler is controlled, such that the coiling speed is increased to accommodate the increased strip speed. In this way the tension is recovered to the original state, such that the set coiling torque matches the tension torque.

However, small differences between roll gap exit strip speed and strip coiling speed already cause big tension variations. These variations can be compensated by anticipating what the new coiling speed has to be and by providing extra torque to the drive of the coiler, so as to accelerate faster. The acceleration needs to be 2 m/s².

When we roll a steel strip of 600 mm wide, using a coiling drive system having an inertia of 200 kgm², the coil having a maximum diameter of 1300 mm, the total inertia will be approximately 1500 kgm². In order to accelerate the drive of the coiler and the coiled strip, an additional torque has to be imposed to the drive of the coiler of about 4500 Nm.

Instead of accelerating the coiler, according to the invention the coiling speed is kept constant and the driving system of the rolling stand is decelerated, so as to keep the speed difference in the coiler to a minimum. The driving system of the rolling mill is usually heavier than the drive of a coiler, and the inertia of such a driving system can be approximately 600 kgm². The required deceleration is the same as the acceleration of the coiler. When a work roll diameter of 300 mm is used, the deceleration needs 8000 Nm less torque. On the other hand, for the increased reduction in the strip leaving the rolling mill extra torque is needed. These changes in torque will balance each other more or less, resulting in an approximately even torque needed for the driving system of the rolling mill when producing strip having a variable thickness in length direction. 

1. Method for producing strip having a variable thickness in its length direction, in particular a repetitive thickness variation, using a stand of a rolling mill, wherein the strip has a thickness D1 when entering the mill stand and a thickness D2 when leaving the mill stand, comprising using the speed of the strip when leaving the mill stand as an input to control the speed of the rolls in the mill stand, and keeping the strip between the mill stand and equipment downstream of the strip at a certain tension, the tension of the strip being used as an input for controlling at least one member of the group consisting of the speed of the mill stand and the speed of the downstream equipment.
 2. The method according to claim 1, wherein a decoiler supplies the strip to the rolling mill and a coiler takes up the rolled strip, wherein a first tension measuring device measures the tension in the strip entering the rolling mill and a second tension measuring device measures the tension in the strip leaving the rolling mill, wherein a speed measuring device measures the speed of the strip leaving the rolling mill, wherein the speed and tension measured with the speed measuring device and the second tension measuring device are used as input to control the rolling speed of the rolling mill.
 3. The method according to claim 2, wherein the speed and tension measured with the speed measuring device, the first tension measuring device and the second tension measuring device are used as input to control the tension in the strip entering the rolling mill using a dancer between the decoiler and the rolling mill.
 4. The method according to claim 2, wherein the speed and tension measured with the speed measuring device, the first tension measuring device and the second tension measuring device are used as input to control the decoiling speed of the decoiler.
 5. The method according to claim 1, wherein the speed of the strip leaving the mill stand is kept approximately constant.
 6. The method according to claim 1, wherein the strip leaving the mill stand is given a repetitive thickness variation having a repeating profile length of at most 4 meter.
 7. The method according to claim 1, wherein a program is provided for determining the thickness D2 in the strip, the program controlling the gap of the mill stand.
 8. The method according to claim 1, wherein a program is provided for controlling the speed of the strip when entering the mill stand.
 9. The method according to claim 1, wherein the rolling mill used is a single stand rolling mill.
 10. Apparatus for producing strip having a variable thickness in its length direction, the apparatus comprising: a stand of a rolling mill, upstream equipment, downstream equipment, a tension measuring device for measuring the tension in the strip leaving the mill stand and/or a speed measuring device for controlling the speed of the strip leaving the mill stand, wherein a dancer or looper has been placed between the upstream equipment and the mill stand to accommodate changes in strip length, and wherein no dancer or looper has been placed between the mill stand and the downstream equipment.
 11. The apparatus according to claim 10, wherein a further tension measuring device has been provided for measuring the tension in the strip entering the mill stand.
 12. (canceled)
 13. The method according to claim 1, wherein the strip's variable thickness in its length direction comprises a repetitive thickness variation. 